Message-ID: <1438851337.4626.72.camel@gmail.com>
Subject: Re: [PATCH] mm: add resched points to
 remap_pmd_range/ioremap_pmd_range
From: Mike Galbraith <umgwanakikbuti@gmail.com>
To: =?ISO-8859-1?Q?J=F6rn?= Engel <joern@purestorage.com>
Cc: Michal Hocko <mhocko@kernel.org>, Spencer Baugh <sbaugh@catern.com>,
        Toshi Kani <toshi.kani@hp.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Fengguang Wu <fengguang.wu@intel.com>, Joern Engel <joern@logfs.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>,
        Shachar Raindel <raindel@mellanox.com>,
        Boaz Harrosh <boaz@plexistor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrey Ryabinin <a.ryabinin@samsung.com>,
        Roman Pen <r.peniaev@gmail.com>, Andrey Konovalov <adech.fo@gmail.com>,
        Eric Dumazet <edumazet@google.com>, Dmitry Vyukov <dvyukov@google.com>,
        Rob Jones <rob.jones@codethink.co.uk>, WANG Chao <chaowang@redhat.com>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        Spencer Baugh <Spencer.baugh@purestorage.com>
Date: Thu, 06 Aug 2015 10:55:37 +0200
In-Reply-To: <20150730165803.GA17882@Sligo.logfs.org>
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	 <20150724070420.GF4103@dhcp22.suse.cz>
	 <20150724165627.GA3458@Sligo.logfs.org>
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	 <20150727151814.GR9641@Sligo.logfs.org>
	 <20150728133254.GI24972@dhcp22.suse.cz>
	 <20150728170844.GY9641@Sligo.logfs.org>
	 <20150729095439.GD15801@dhcp22.suse.cz>
	 <1438269775.23663.58.camel@gmail.com>
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On Thu, 2015-07-30 at 09:58 -0700, Jörn Engel wrote:
> On Thu, Jul 30, 2015 at 05:22:55PM +0200, Mike Galbraith wrote:
> > 
> > I piddled about with the thought that it might be nice to be able to
> > sprinkle cond_resched() about to cut rt latencies without wrecking
> > normal load throughput, cobbled together a cond_resched_rt().
> > 
> > On my little box that was a waste of time, as the biggest hits are block
> > softirq and free_hot_cold_page_list().
> 
> Block softirq is one of our problems as well.  It is a bit of a joke
> that __do_softirq() moves work to ksoftirqd after 2ms, but block softirq
> can take several 100ms in bad cases.

On my little desktop box, one blk_done_softirq() loop iteration can take
up to a few milliseconds, leaving me wondering if breaking that loop
will help a studly box much.  iow, I'd like to know how bad it gets, if
one iteration can be huge, loop breaking there is fairly pointless, and
I can stop fiddling.  Do you happen to know iteration time during a size
huge block softirq hit?  On my little box, loop break/re-raise and
whatnot improves the general case substantially, but doesn't do much at
all for worst case.. or rather the next worst case in a list of unknown
length ;-)

	-Mike

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